The objective is to characterize the activities of cranial motor nerves which innervate upper airway muscles and spinal nerves innervating muscles of the chest wall and diaphragm. These characterizations are important since patency of the upper airway requires an appropriate balance between these cranial and spinal nerve activities. Studies will be concerned with pulmonary reflex mechanisms. The influence of both the phasic and tonic discharge of pulmonary stretch receptors will be examined. The tonic discharge is hypothesized to be a major determinant of respiratory-modulated cranial motoneuronal activity during expiration. Moreover, the level of excitability of brainstem respiratory neurons is influenced by the tonic discharge of pulmonary stretch receptors. This level of excitability determines motoneuronal responses to peripheral chemoreceptor stimulation in hypoxia. Concerning the cranial motoneurons, their activity is influenced by the "history" of phasic activity of pulmonary stretch receptors; this history will be characterized. Another aspect of pulmonary stretch receptor discharge, its facilitation of inspiratory activity, is a continuous determinant of phrenic motoneuronal activity. Phrenic activity can also be inhibited by the discharge of pulmonary stretch receptors. Conditions for the expression of a new facilitation or inhibition will be defined. This definition will allow for a critical evaluation of the roles of I-alpha and I-beta groups of medullary respiratory neurons. Their discharge patterns are hypothesized to underlie the inhibition and facilitation of phrenic activity by the discharge of pulmonary stretch receptors. Thus, I-alpha and I-beta neurons are hypothesized to represent a continuum.